PaperMC/patches/server/0665-Fix-swamp-hut-cat-generation-deadlock.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Spottedleaf <Spottedleaf@users.noreply.github.com>
Date: Sat, 12 Mar 2022 06:31:13 -0800
Subject: [PATCH] Fix swamp hut cat generation deadlock

The worldgen thread will attempt to get structure references
via the world's getChunkAt method, which is fine if the gen is
not cancelled - but if the chunk was unloaded, the call will block
indefinitely. Instead of using the world state, we use the already
supplied ServerLevelAccessor which will always have the chunk available.

diff --git a/src/main/java/net/minecraft/world/entity/animal/Cat.java b/src/main/java/net/minecraft/world/entity/animal/Cat.java
index 93d8d45a9a8108e58f3a96d77dd35c7584133835..629b282cf27806ff37d67f83d44c06a9f32a9185 100644
--- a/src/main/java/net/minecraft/world/entity/animal/Cat.java
+++ b/src/main/java/net/minecraft/world/entity/animal/Cat.java
@@ -365,7 +365,7 @@ public class Cat extends TamableAnimal implements VariantHolder<Holder<CatVarian
         BuiltInRegistries.CAT_VARIANT.getRandomElementOf(tagkey, world.getRandom()).ifPresent(this::setVariant);
         ServerLevel worldserver = world.getLevel();
 
-        if (worldserver.structureManager().getStructureWithPieceAt(this.blockPosition(), StructureTags.CATS_SPAWN_AS_BLACK).isValid()) {
+        if (worldserver.structureManager().getStructureWithPieceAt(this.blockPosition(), StructureTags.CATS_SPAWN_AS_BLACK, world).isValid()) { // Paper - Fix swamp hut cat generation deadlock
             this.setVariant((Holder) BuiltInRegistries.CAT_VARIANT.getOrThrow(CatVariant.ALL_BLACK));
             this.setPersistenceRequired();
         }
diff --git a/src/main/java/net/minecraft/world/level/StructureManager.java b/src/main/java/net/minecraft/world/level/StructureManager.java
index 3d607ac74de4bc051ab68269dbab0bbd3a52dc54..03adb02297911e5a5051edac14453d7e3eeac29c 100644
--- a/src/main/java/net/minecraft/world/level/StructureManager.java
+++ b/src/main/java/net/minecraft/world/level/StructureManager.java
@@ -48,7 +48,12 @@ public class StructureManager {
     }
 
     public List<StructureStart> startsForStructure(ChunkPos pos, Predicate<Structure> predicate) {
-        Map<Structure, LongSet> map = this.level.getChunk(pos.x, pos.z, ChunkStatus.STRUCTURE_REFERENCES).getAllReferences();
+        // Paper start - Fix swamp hut cat generation deadlock
+        return this.startsForStructure(pos, predicate, null);
+    }
+    public List<StructureStart> startsForStructure(ChunkPos pos, Predicate<Structure> predicate, @Nullable ServerLevelAccessor levelAccessor) {
+        Map<Structure, LongSet> map = (levelAccessor == null ? this.level : levelAccessor).getChunk(pos.x, pos.z, ChunkStatus.STRUCTURE_REFERENCES).getAllReferences();
+        // Paper end - Fix swamp hut cat generation deadlock
         Builder<StructureStart> builder = ImmutableList.builder();
 
         for (Entry<Structure, LongSet> entry : map.entrySet()) {
@@ -116,10 +121,20 @@ public class StructureManager {
     }
 
     public StructureStart getStructureWithPieceAt(BlockPos pos, Predicate<Holder<Structure>> predicate) {
+        // Paper start - Fix swamp hut cat generation deadlock
+        return this.getStructureWithPieceAt(pos, predicate, null);
+    }
+
+    public StructureStart getStructureWithPieceAt(BlockPos pos, TagKey<Structure> tag, @Nullable ServerLevelAccessor levelAccessor) {
+        return this.getStructureWithPieceAt(pos, structure -> structure.is(tag), levelAccessor);
+    }
+
+    public StructureStart getStructureWithPieceAt(BlockPos pos, Predicate<Holder<Structure>> predicate, @Nullable ServerLevelAccessor levelAccessor) {
+        // Paper end - Fix swamp hut cat generation deadlock
         Registry<Structure> registry = this.registryAccess().lookupOrThrow(Registries.STRUCTURE);
 
         for (StructureStart structureStart : this.startsForStructure(
-            new ChunkPos(pos), structure -> registry.get(registry.getId(structure)).map(predicate::test).orElse(false)
+            new ChunkPos(pos), structure -> registry.get(registry.getId(structure)).map(predicate::test).orElse(false), levelAccessor // Paper - Fix swamp hut cat generation deadlock
         )) {
             if (this.structureHasPieceAt(pos, structureStart)) {
                 return structureStart;